Multistage rotary pumps



Dec. A8., 197.0 y H. wYcLlFr- ETAL u 3,545,888

MULTISTAGE ROTARY PUMPS 3 Sheets-Sheet 1 Filed Sept. 16, 1968 'INVENTOR'S v BY 7M* fu@ ATTORNEY Dec. 84, 1970 H. wYcLlFFE ET AL 3,545,838

MULTISTAGE ROTARYl PUMPS V 5 Sheets-Sheet 2 Filed Sept. 16, 1968 xNvENToRs BY //f/ymw lATTORNEY Dec. 8., 1970 H. WYCLIFFE: ET AL 3,545,888

` L MULTISTAGE ROTARY PUMPS Filed Sept. 16, 1968 3 Sheets-Sheet 5 INVENTOR:

BY a Mya@ ATTORNEY United States Patent O 3,545,888 MULTISTAGE ROTARY PUMPS Henryk Wyclitfe, Crawley, and Basil Dixon Power,

Horsham, England, assignors to Edwards High Vacuum International Limited, Crawley, Sussex, England, a

British company Filed Sept. 16, 1968, Ser. No. 759,955 Int. Cl. B01d 39/00; F04c 23/00; F25j 3/00 U.S. Cl. 417--310 Claims ABSTRACT OF THE DISCLOSURE A multistage rotary vacuum pumping arrangement in which a low vacuum stage is oil lubricated and a high vacuum stage is dry or non-lubricated in the conventional sense. The dry stage is driven in phase to follow the lubricated stage. An oil vapour trap is suitably situated in an interstage region.

This invention relates to multistage rotary vacuum pumps.

According to the present invention a multistage rotary vacuum pumping unit includes a lubricated low vacuum stage, a non-lubricated high vacuum stage and means connecting the said two stages whereby relative movement between co-operating pumping components of the low vacuum stage results in driven corresponding in-phase movement of co-operating pumping components in the high vacuum stage.

Preferably the two stages are of identical type but this is not essential as long as the high vacuum stage pumping components are caused to be driven in phase with the pumping components of the low vacuum stage.

Wherever the term co-operating is used in this specification and the appended claims and it relates to pumping components, it will be understood that the components do not necessarily co-operate, and indeed sometimes do not, in a physical sense. The term is to mean co-operation to produce the required pumping effect. In certain cases it is desirable that the non-lubricated components of the high vacuum stage do not contact each other but, since wear of the pumping components of the low vacuum stage might cause occasional dry Contact, the nonlubri cated pumping components may be self-lubricating or have some form of dry lubrication. The term nonlubricated refers to the absence of an organic lubricant, such as silicone oil. In other cases however, especially when an organic vapour trap is placed between the stages, a high compression ratio is required in the high vacuum stage. There should then preferably be contact between the non-lubricant components and they can then suitably comprise polytetrauorethylene coated components.

Another multistage rotary pump of which is suited to the present invention is a composite unit in which both stages comprise pumps of the screw type.

It will thus be seen that a multistage pump can be constructed in accordance with the invention so that the low vacuum stage can be oil sealed and oil lubricated, which makes it suitable for pumping directly to atmosphere, but that thehigh vacuum stage is dry and the tendency of oil vapours migrating back to the high vacuum side is, to a large extend overcome. The performance in this respect can be improved still further by placing a trap for organic vapours between the high and low vacuum stages. Such a trap may take the form of a refrigerated surface or it may contain an absorbent, for instance, activated alumina.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a longitudinal section of' a two stage pump constructed in accordance with the invention;

3,545,888 Patented Dec. 8, 1970 FIG. 2 shows a transverse section along line A-A of FIG. 1;

FIG. 3 shows a valve arrangement which may be employed in the interstage passage of the pump illustrated in FIGS. 1 and 2; and

FIG. 4 shows a two stage pump of the screw type constructed in accordance with the invention.

Referring now to FIGS. 1 and 2, a vertically arranged two stage vacuum pump has an inlet 1 for connection to the region to be evacuated and comprising the inlet to the high vacuum stage 2 of the pump. The outlet from the high Vacuum stage 2 is provided with an interstage passage 3 which leads to an interstage trap 4. This trap may be iilled with activated alumina so that it acts by sorption to inhibit migration of oil Vapour towards the high vacuum stage. A further passage 3a leads away from the trap 4 and to the inlet 5 of the low vacuum stage 6 which has an exhaust valve 7, shown diagrammatically in FIG. 2.

The low vacuum stage 6 has a rotor 8 which is eccentric about a rotatable shaft 9 to which it is keyed. A follower 10 is keyed to a shaft 11 and biased to bear against the rotor 8 by a torsion spring 12 acting upon the shaft. The pumping action is achieved by rotation of shaft 9 in an anti-clockwise sense as viewed in FIG. 2, shaft 11 oscillating due to the follower 10 bearing on the surface of the eccentric rotor 8. The two shafts 9 and 11 are each provided with a pair of bearings 13 and 14 respectively beyond each of which is an oil seal 15. These seals define the end of the low vacuum oil sealed pumping stage 6, the seals 15 inhibiting the oil from penetrating the high vacuum stage 2.

The high vacuum stage 2 is identical in most respects to the low vacuum stage 6 in transverse section. It thus includes a rotor 16, identical to rotor 8 and keyed to shaft 9, and a follower 17 which is identical to follower 10, also being keyed to shaft 11. The main difference between the two stages is that stage 2 is dry While stage 6 is oil lubricated and oil sealed. On account of this and assuming the corresponding rotors and followers are identical, follower 10 will always be separated from the rotor 8 by the thickness of a iilm of lubricant, whereby follower 17 will follow follower 10 and be ideally separated from rotor 16 by a gap equal to the oil film thickness. Since the high vacuum dry stage is normally associated with a small gap, it will be appreciated that, for a proper working of the pump unit, the low vacuum stage must be capable of delivering to atmosphere from a pressure at which the mean free path of the molecules of the medium being pumped must be at least as large as the said gap to limit leak back. The followers 10 and 17 can be a light material, such as a laminated plastics material or aluminum, to minimise vibration.

Bearings 13 and 14 are situated within the low vacuum stage 6 and between the oil seals 15 so that they can be lubricated appropriately. However it is essential that lubricant is prevented from entering the high vacuum stage. Thus should any oil escape along either of the shafts 9 or 11 past the oil seals 15, it is centrifuged by throw rings 18 beyond each of which and surrounding the corresponding shaft is an annulus of sorbent material 19. Moreover, the rotor 16 and follower 17 can be overhung on their respective shafts to overcome the problem of additional bearings in the high vacuum stage. Should the stage be long, dry bearings can be used or alternatively bearings lubricated with grease if they are shaft sealed and maintained at a rough vacuum by connecting the bearing side of the seals to the inlet 5 of the low vacuum stage. By way of illustration, FIG. 1 shows rotor 16 overhung on its shaft 9 and the top end of shaft 11 supported in a dry bearing 20.

The sorbent trap 4 is placed between the two stages and, as mentioned earlier, is suitably filled with activated alumina. When such a trap is positioned at the inlet of a pump, it impedes flow to the inlet and thus reduces the effective pump capacity. Moreover, during evacuation ,the trap absorbs moisture which, at a later stage in pumping, exerts water vapour pressure to limit the ultimate vacuum attainable and prolongs pump down. These advantages are largely overcome by positioning the trap between the two stages, as shown in FIG. l. The pumping capacity is not reduced by the same extent as when the trap is at the pump inlet and the water vapour pressure, referred to above, is supressed by the pumping action of the dry stage 2. Although the trap is shown in FIG. Kl as being external to the pump, it could be provided in the space 21. In order to guard against water vapour escaping to the high vacuum side of the unit, the pumping components of the high vacuum stage 2 may be polytetrafluorethylene coated and driven in dry contact.

A further feature designed to reduce absorption of moisture in the trap 4 is the provision of a relief bypass valve 22 positioned near the outlet of the high vacuum stage 2 and a flow control valve 23 in the interstage passage 3. FIG. 3 shows the interstage passage 3 to have two regions, 3b and 3c, which are downstream and upstream respectively of the iiow control valve 23. The by-pass valve 22 functions to deliver most of the air handled during pump-down of a system directly to atmosphere, the trap 4 thus being by-pa'ssed. The valve 22 is shown in FIG. 3 as comprising a valve plate 25 having a washer or rubber ring 26 and provided with a seating 27. The valve plate 25 is biased to its closed position by the combination of atmospheric pressure and a spring 24. During the initial stages of pump-down the interstage pressure in region 3c is sufficient to overcome the combined forces of the atmosphere and spring 24 so that the valve opens and the pumped medium in region 3c can be delivered directly to atmosphere. The valve is positioned near the outlet port of the high vacuum stage.

The ow control valve comprises a diaphragm 28 adapted under differential pressure to cover the entrance to the region 3b of the interstage passage 3. The diaphragm has a central aperture 29 which permits limited passage of air towards the low vacuum stage. During the early stages of evacuation sufficient pressure differential exists across diaphragm 28 to deflect it and close the passage 3b which leads to trap 4. The only ow allowed through the trap and the low vacuum stage is that restricted flow through the aperture 29. As evacuation progresses, the amount of air handled diminishes whilst the low vacuum stage continues pumping through aperture 29. The pressure differential across the diaphragm will diminish and the diaphragm 8 will then return to the position shown in FIG. 3 exposing region 3b of the interstage passage 3 for normal pumping.

Clearly the valves 22 and 23 may be of any suitable type. Moreover, the flow control valve 23 could be arranged completely to isolate the inlet to the trap 4 during the initial stage of pump-down, this inlet being opened when two stage working is required. Suitably a bellows operated valve with the interior of the bellows connected to the pump inlet would fulfill this function.

FIG. 4 shows a composite pumping unit comprising a pair of pumps of the screw rotor type. The unit is provided with a housing 30 which has a flange 31 for connection to a system to be pumped and a port 32 for discharge from the unit of the pumped medium. Mounted in bearings 33 at the base of the unit and in bearings 34 intermediate its ends is a pair of shafts 35 and 36. Bearings 34 are at an interstage position and may be similar to the interstage bearing arrangements already described with reference to FIG. l. The upper ends of the shafts are overhung although dry bearing or bearings suitably protected from the high vacuum side of the unit. The shaft 34 passes through the base of the housing 30 so that it can be driven by means (not shown).

Mounted on the shafts are two pairs of rotor gear elements, 37, 38 and 39, 40, each of which pairs cooperate to produce a pumping action when they rotate in opposite direction. The top ends of the rotor pairs are partially covered by cover plates 41 and 42, the noncovered or exposed areas defining inlet ports which allow flow of the pumping medium into the spaces defined by the rotors and the housing 30. When opposed rotation of the rotors of each pair takes place the medium being pumped is enclosed within these working spaces which move axially along the rotors towards an outlet port in the lower end covers 43 or 44. As each space approaches an outlet port in the associated end cover the interaction of the rotors reduces its volume.

Rotors 37 and 38 form the high pressure oil sealed stage, the latter rotor being driven by the former. The oil is introduced to the working spaces after they are sealed from the interstage region by interaction of the rotors, the leakage path therefore including at least one working clearance restriction 45 between the rotors and the housing 30. In the region below the oil introduction points the rotors 37 and 38 bear against each other, but they are lubricated by the sealing oil. Above this region the working clearances should be sufficient to prevent any undue rubbing and consequent friction Wear since there is no lubricant. Particular desirable is the use of an arrangement of outlet valves co-operating with the outlet end cover 43 which closely abuts the end faces of the rotor pair and the housing so as to have a very small clearance or to rub in lubricating contact with the rotors. A first set of outlet ports having associated one-way valves are placed so as to permit the discharge of pumped gas as soon as the pressure in the working chambers exceeds the pressure against which the pump is discharging (e.g. atmosphere) when there is a considerable throughput of gas during the early stages of pumping. There is also another set of valves and associated ports so positioned that they are connected to the working chambers when the latter are reaching their minimum size towards the end of compression. These ports and valves generally correspond to the illustrated port 46 and will be positioned in the end cover plate over the region of the nal inter-lobal space defining the working chamber. There may be a. single port with a valve, a number of ports leading to a single valve or a number of ports leading to respective valves. 'Ihe oil metered into the pump ensures that, in the final stage of delivery, the working chambers are almost completely filled with oil, any pumped gas being compressed to a very small volume, and when the outlet valve or valves are lifted the oil is caused to flow through the ports in the manner in a hydraulic piston ensuring very complete elimination of gas at the end of each compression cycle.

All the outlet valves of the oil sealed pump may be oil immersed so as to prevent any back flow of gas into the working chamber if the latter should be at a lower pressure than the discharge pressure.

Rotors 39 and 40 comprise the non-lubricated high vacuum stage and are driven, in accordance with the invention, by the rotors 37 and 38. This high vacuum stage is dry and the working clearances 47 must be of the order of 0.005 in., while such clearances are smaller than the mean free path of gas molecules at high vacuum, it is desirable in order to reduce leak back rate from the interstage region to ensure that they are also comparable or less than the mean free path of the pumped gas molecules in that region. An outlet port 48 in cover plate 44 need not be provided with a valve. However, during initial evacuation of the system to be pumped the volume of gas contained within each working chamber will tend to be compressed until its pressure considerably exceeds that existing between the stages. This would require undesirably large amounts of power and probably cause rough Working of the pump. Thus, there is provided, in addition to the outlet port 48, one or more additional outlet ports (not shown) which communicate with the working chambers at various stages of their reduction in volume. These additional ports are provided with one- Way valves, conveniently flap valves, which only open when the pressure in the working chamber exceeds that in the interstage region by a predetermined amount, say 3 p.s.i. Thus, while these valves will be in use during the initial stages of pumping, they Will not necessarily be used when high vacuum is reached. The final outlet port 14 may, as mentioned above, also be provided with a valve if wished, though this is not normally necessary.

As an alternative and preferred manner of driving the Unit the shafts 35 and 36 may be provided with synchronising gears (not shown). This removes the possibility of rotor wear in the lubricated stage and thus permanently ensures exact in-phase movement of the rotors of the two stages.

We claim:

1. A multistage rotary mechanical vacuum pumping arrangement including in combination:

(a) a lubricated low vacuum pumping stage having first and second co-operating pumping components;

(b) a dry higher vacuum pumping stage having third and fourth co-operating pumping components and means whereby said third and fourth pumping cornponents co-operating predominantly without contact therebetween; and

(c) connection drive means between said two stages.

2. An arrangement according to claim 1, in which said two stages comprise pumps of an identical type, said connection drive means connecting said rst and third components together to drive in unison and connecting said second and fourth components also to drive in unison.

3. An arrangement according to claim 1, including an interstage region and an organic vapour trap in said region.

4. An arrangement according to claim 3, in which said trap contains activated alumina.

5. An arrangement according to claim 3, including an excess pressure by-pass valve situated between said interstage region on the high vacuum side of said trap and atmosphere.

6. An arrangement according to claim 5, including a tiow control valve situated in said interstage region on the high vacuum side of said trap.

7. An arrangement according to claim y6, in which said flow control valve has a closure member comprising an apertured diaphragm.

8. An arrangement according to claim 2, in which said rst component and said third component comprise rotors rotatable about a common eccentric axis, said rotors being keyed to a first rotatable common shaft, and said second component and said fourth component comprise rotor followers keyed to a second common rotatable shaft which is biased to hold said second component in contact with said rst component.

9. An arrangement according to claim 2, in which each of said pumping stages comprises a pump of the screw rotor type.

10. A multistage rotary mechanical vacuum pumping unit including in combination:

(a) a lubricated low vacuum pumping stage having a rst pumping component and a second co-operating pumping component;

(b) a dry higher vacuum pumping stage of a type identical to said lubricated stage and having a third pumping component and a fourth co-operating pumping component, said third component corresponding to said iirst component and said fourth component corresponding to said second component;

(c) a rst common shaft to which both said first component and said third component are keyed;

(d) a second common shaft to which both said second component and said fourth component are keyed;

(e) an interstage region through which the medium to be pumped ows;

(f) an organic vapour trap situated in said region;

(g) an excess pressure by-pass valve situated between said region on the high vacuum side of said trap and atmosphere; and

(h) a flow control valve situated in said region on the high vacuum side of said trap.

References Cited UNITED STATES PATENTS 2,636,655 4/1953 McFee 230-45 2,935,243 5/1960 Sadler 230L-45 2,968,160 1/1961 Schilling et al 62-18 3,007,624 11/1961 Netzel 230-45 3,085,913 4/1963 Caswell 230-45 3,200,569 8/1965 Wheeler 230-69 3,283,479 ll/l966 Batzer et al. -389 3,438,570 4/1969 Bode et al. 2304-158 HENRY F. RADUAZO, Primary Examiner U.S. Cl. X.R. 

